In recent years, a technique for manufacturing a thin film transistor (hereinafter referred to as a TFT) over a substrate has drastically progressed and development for applying a TFT to an active matrix display device has been advanced. In particular, since a TFT using a polycrystalline semiconductor film has higher electric field effect mobility (also referred to as mobility, simply) than a conventional TFT using a non-single crystal semiconductor film, high speed operation is possible. Therefore, it has been attempted that a pixel, which has been conventionally controlled by a driver circuit provided outside a substrate, is controlled by a driver circuit provided over the same substrate as the pixel.
A substrate used for a semiconductor device is expected to be a glass substrate rather than a quartz substrate in terms of cost. However, a glass substrate is inferior in heat resistance and easy to be deformed due to heat. Therefore, when a TFT using a polycrystalline semiconductor film is formed over a glass substrate, laser annealing is often employed for crystallizing a semiconductor film in order to prevent the glass substrate from being deformed due to heat.
Compared with another annealing method which uses radiation heat or conduction heat, the laser annealing has advantages such that the process time can be shortened drastically and that a semiconductor substrate or a semiconductor film can be heated selectively or locally so that thermal damage is hardly given to the substrate.
In this specification, the laser annealing method means a technique of crystallizing a damaged layer or an amorphous layer formed in a semiconductor substrate or a semiconductor film, a technique of crystallizing an amorphous semiconductor film formed over a substrate. Moreover, the laser annealing includes a technique applied to planarize or modify a surface of a semiconductor substrate or a semiconductor film.
In this specification, when a laser irradiation position on a semiconductor substrate is determined with accuracy, a method is employed, in which a marker serving as a reference is provided on an irradiation surface and the irradiation position is controlled based on the marker by a video processing means including a CCD camera, a personal computer or the like (for example, Reference 1: Japanese Published Patent Application No. 2003-224084).